I've been setting up a HTPC on a new MacMini. Nothing fancy, it's basically a MacMini running Plex, which (if you're not familiar with it) is a free media server application similar to XBMC and Boxee.
I used Plex about a year and a half ago, when it was in rougher stages. Today, it seems both more robust as an application and also offers more variety in terms of the content available.
In addition to stuff like Netflix, TED, and South Park, there are "channels" from Snag Films and the National Film Board of Canada, which has an amazing library of films online, including works by Michel Brault.
Vimeo's HD channel looks amazing, too -- it looks as good as any HD cable I've seen. So far there's no Mubi support. Hopefully soon.
You don't need Plex to watch these videos, of course. Click the image below to watch Pour la suite du monde (aka Of Whales, the Moon, and Men).